This proposal is based on two remarkable findings from our lab. First, complement neutralizes the closely related paramyxoviruses Simian Virus 5 (SV5) and Mumps Virus (MuV), but by distinct mechanisms involving formation of massive SV5 aggregates versus complement mediated lysis of MuV particles. Secondly, our microarray data indicate that infection with the noncytopathic WT SV5 results in elevated levels of host cell regulators of complement and these virus-infected cells are not lysed by complement. Our long term goal of this project is to understand the cellular and viral factors that dictate the outcome of interactions of paramyxoviruses with complement. The short term goals are to 1) utilize a novel experimental system to identify viral signals that dictate the extent and pathway of complement neutralization and to 2) define the role of SV5-induced regulators of complement in preventing lysis of virus infected cells. An innovative aspect of our application is the use of SV5 and MuV virus-like particles (VLP) to analyze virus:complement interactions. This is a powerful and flexible system, in that novel VLPs that differ in viral components can be produced when these alterations would otherwise not be tolerated in the context of a bone fide infectious virus. In Aim 1, we will identify the cellular pathways and viral determinants involved in interactions of complement with SV5 and MuV virus-like particles (VLPs). The complement pathways and the viral determinants of complement activation will be identified through the use of VLPs that differ in their content of viral glycoproteins HN, F and SH. In Aim 2, we will test the hypothesis that SV5-infected epithelial cells are resistant to complement-mediate lysis due to upregulation of host cell regulators of complement. There is intense interest in defining mechanisms that modulate the interplay between viruses and the host innate immune system, but the role of complement in neutralization of paramyxoviruses is not completely understood. Our studies have important implications for understanding complement-mediated neutralization of viruses, the re-emergence in the human population of paramyxoviruses such MuV, and persistent infections, as well as great potential for exploiting the new information for novel therapeutic applications. PUBLIC HEALTH RELEVANCE: There is intense interest in defining mechanisms that modulate the interplay between viruses and the host innate immune system, but the role of complement in neutralization of paramyxoviruses is not completely understood. Our work will have important implications for control of viruses such as MuV, which has re-emerged despite wide-spread vaccination. In addition, our work with virus-induced inhibition of complement has implication for mechanisms by which viruses establish persistent infections that are refractory to recognition by host immune systems.